1. Field of the Invention
The present invention relates to a torque distribution control system for a four wheel drive vehicle.
2. Description of Related Art
For a four wheel drive vehicle adapted to drive all of the four wheels, output torque generated by the engine is distributed to the four wheels and then transmitted to pavement so that it can afford greater tire performance than a two wheel drive vehicle, thereby providing propulsive force more effectively than the two wheel drive vehicle. From this point of view, there is the increasing tendency that the four wheel drive vehicle is adopted for passengers cars and sports cars as well as off-road cars.
For the four wheel drive vehicle, the setting of a torque distribution ratio of each of the wheels exerts a great impact upon vehicle performance. In instances where acceleration is required particularly at the time of starting or accelerating the vehicle, on the one hand, it is desired that the torque distribution ratio of the rear wheels is rendered greater than the ratio of the front wheels because a load applied to the rear wheels is heavier than that to the front wheels. When the vehicle is cornering or is driven straight, the torque distribution ratio of the front wheels is desired to be greater than that of the rear wheels.
Recently, four wheel drive vehicles are on the market, which are provided with a torque distribution adjusting means for altering a torque distribution ratio between the front and rear wheels so as to change the torque distribution ratio of the front wheels to the rear wheels in accordance with various driving states such as starting, straight driving, cornering or the like. Japanese Patent Unexamined Publication (kokai) No. 248,440/1985 discloses technology which involves preventing a drive torque from flowing out from a wheel which causes slipping by braking the slipping wheel. As it is commonplace that the four wheel drive vehicle is provided with a differential gear between each of the wheels, for example, if the left-hand rear wheel slips, there is the problem that all of the output torque of the engine is flown out from the left-hand rear wheel and it is not transmitted at all to the other wheels. The technology as disclosed in the above prior patent publication is to prevent an outflow of the output torque of the engine from the wheel which causes slipping.
Japanese Patent Unexamined Publication (kokai) No. 268,529/1986 discloses a means for judging a wheel which slips. As a matter of course, the four wheel drive vehicle has no undriven wheel so that the vehicle velocity cannot be determined on the basis of the peripheral velocities of the undriven wheels for the four wheel drive vehicle, unlike the two wheel drive vehicle. In the technology disclosed in this prior patent publication an estimated vehicle velocity is determined on the basis of a velocity of each wheel when acceleration of each wheel exceeds a given reference value, and a minimum value is selected from the estimated vehicle velocity and the velocity of each wheel. Then the wheel having its velocity greater than the minimum value is judged as a slipping wheel.
In Japanese Patent Unexamined Publication (kokai) No. 41,245/1988, it is disclosed that technology involves altering a torque distribution ratio of the front wheels to the rear wheels by controlling a friction clutch so as to decrease torque to e transmitted to the wheel that causes slipping due to acceleration of the vehicle, the friction clutch being disposed each to a front propeller shaft for transmitting the output torque of the engine to the front wheels and to a rear propeller shaft for transmitting the output torque thereof to the rear wheels. The technology disclosed in this prior patent publication, however, presents the problem that responsiveness to convergence of a slip may be impaired because the friction clutch disposed at each of the propeller shafts is controlled in order to prevent a slip of the wheel. If one of the wheels slips, operation of the brake disposed at the slipping wheel is most effective to converge the slip and is best in responsiveness to slip convergence. As long as the responsiveness to slip convergence is concerned, the technology as disclosed in Japanese Patent Unexamined Publication (kokai) No. 248,440/1985 is better. This technology, however, braking causes a loss of energy on the other hand. In other words, braking may radiate a portion of its drive torque as a heat energy, thus causing the problem that a drive power cannot be provided as required by the driver.